The invention relates to a method and a master for producing a volume hologram, a security element with such a volume hologram, as well as a security document with such a security element.
Holograms are used as security elements to protect security documents such as banknotes, passports, security cards or the like in order to achieve a high degree of protection against forgery. For mass-produced articles, surface relief holograms are often used which on the one hand do not generate an optimal image impression and on the other hand can be copied by molding the surface relief.
Volume holograms, also referred to as white-light holograms or reflection holograms, are usually produced by means of a master hologram to be exposed to light, depend on the light diffraction at the so-called Bragg planes of a transparent layer, whereby the transparent layer has local differences in refractive index, and generate a brilliant, but monochromatic image impression. They cannot be copied by molding a surface relief.
If volume holograms with several differently colored areas are to be produced, several masters must generally be used for the exposure of the volume hologram to light of different wavelength. When exchanging the masters, positioning inaccuracies necessarily occur which reduce the quality of the volume hologram.